1. Field of the Invention
The invention relates to a fixing device for heating and fixing an unfixed image formed and borne on a recording material, and an image forming apparatus such as a printer, a copier or a facsimile apparatus using the fixing device. More particularly, the invention relates to a fixing device of the film heating type or the pressure rotary member driving type, and an image forming apparatus provided with the fixing device. The fixing device of the film heating type will hereinafter be sometimes referred to as a film type fixing device.
2. Description of Related Art
In image forming apparatuses such as a printer, a copier and a facsimile apparatus using the image forming technique such as the electrophotographic process or the electrostatic recording process, heat roller fixing devices of the contact heating type good in heat efficiency and safety and energy saving type and on-demand type fixing devices of the film heating type have heretofore been widely adopted as fixing devices for fixing an unfixed image formed and borne on a recording material.
A fixing device of the film heating type is proposed, for example, in Japanese Patent Application Laid-Open No. 63-313182, Japanese Patent Application Laid-Open No. 2-157878, Japanese Patent Application Laid-Open Nos. 4-44075 to 44083, Japanese Patent Applications Laid-Open Nos. 4-204980 to 204984, etc., and is a device having a heater (heating member), heat-resistant film (fixing film) frictionally sliding relative to this heater, and a pressure member being in pressure contact with the heater with the film interposed therebetween and forming a fixing nip portion, and in which a recording material having an unfixed image formed thereon is nipped and conveyed between the film and the pressure member in the fixing nip portion, whereby the unfixed image on the recording material is a fixed as a permanent image by heat imparted from the heater through the film and the pressure force of the fixing nip portion.
Such a fixing device of the film heating type can use a low heat capacity linear heating member as the heater, and use thin film of low heat capacity as the film and therefore, the saving of electric power and the shortening of waiting time (the quick starting property) are possible.
Also, in the fixing device of this film heating type, as driving systems for the fixing film, there are a system using a conveying roller and a driven roller exclusively for the conveyance of the film and conveying the fixing film between the conveying roller and a pressure roller as the pressure member while applying tension, and a tensionless system for rotatively driving the pressure roller as the pressure member to thereby drive cylindrical fixing film by the conveying force of the pressure roller. The former has the advantage that the conveying property of the fixing film can be heightened, and the latter has the advantage that the construction of the device can be simplified to thereby realize a low-cost device. In recent years, the latter rotary member driving system for pressurization has often been adopted because of the small number of parts required in it.
Also, with the recent development of the computer industry, the demand for printers has been on the rise and printers have come to be used in various countries of the world. Thus, recording materials used are rich in the variety of type, thickness, surface property, etc. and at the same, coupled with the higher speed of the image forming apparatus, the instantaneous quantity of heat given from the heater to the recording material has been increased little by little to thereby obtain a satisfactory fixing property from the viewpoints of the shortening of the printing time for the first sheet, the securement of the fixing property of the first printed sheet, etc. Further, in recent years, the user""s requirement for a high quality of image has increased and printers excellent in dot reproducibility and halftone reproducibility have been announced, and the particle diameter of a toner which is a visualizing agent (developer) has been made smaller, whereby a higher image of quality has been achieved.
When use was made of developing devices of the film heating type and the pressure rotary member driving type and 50 sheets were passed in intermittent printing, xe2x80x9cslip jamxe2x80x9d occurred in the fixing devices.
The slip jam (hereinafter referred to as the slip) is a phenomenon which is caused by the pressure roller and a recording material (hereinafter referred to as the paper sheet) bearing an unfixed image thereon slip relative to each other when in a fixing device of the film heating type and the pressure rotary member driving type which is pressure roller driving and film driven (tensionless), the paper sheet has been conveyed to the fixing nip portion.
The result of the measurement of the rotational speed of the film effected by a laser Doppler speed meter in a state in which the slip occurred is shown in FIG. 14 of the accompanying drawings. As shown in FIG. 14, it will be seen that from the moment when the paper has been nipped by the fixing nip portion, the paper slips, whereby the speed of the film becomes lower and the driving force from the pressure roller is not transmitted to the film. Therefore, the paper sheet cannot be conveyed at a constant speed and the slip occurs.
Further, when the confirmation of the slip has been made, it has been found that the slip is liable to occur under the following conditions.
Condition (i): the moisture absorbing amount of the recording material (paper sheet) is high.
Condition (ii): the temperature of the pressure roller is high.
Condition (iii): the dot percentage of the print pattern is high.
About Condition (i)
Media various in texture, basis weight, etc. are used for printing. Among these, there are media which absorb moisture when they are left under a high-temperature high-humidity environment. Therefore, when in the course of printing, the paper is conveyed to the fixing nip, the pressure roller and the film are sufficiently warmed by the heating of the heater, and when the paper sheet is nipped by the fixing nip, steam is created from the paper. The steam created at this time is present between the pressure roller and the paper sheet and therefore, the conveying force of the pressure roller is not transmitted to the film through the paper and the film slips.
The heater is heating also during the time when the film slips, and by time elapsing after the slip has occurred, the steam contained in the paper evaporates, and when the driving force from the pressure roller comes to be transmitted to the film through the paper sheet, the paper conveying speed comes to be obtained stably (FIG. 14). However, by the slip occurring at first, paper conveyance is delayed to thereby cause jam.
Generally, as paper liable to cause slip, mention may be made of paper having the moisture content of 8.0% or greater when left under 23xc2x0 C. and 70% RH, and 70 g/m2 or less as the basis weight.
About Condition (ii)
The fixing temperature during printing (target temperature) is determined so that the fixing property can be secured and heat offset may not occur. Further, the fixing temperature (target temperature) is determined in conformity with the temperature detected by a thermistor immediately before printing is started, and is controlled so as to become a high heater temperature in a state in which the fixing device has been cooled as when a power source switch has been closed, and further, when printing is continued, the pressure roller, a film guide, a rigid pressure stay (a T-stay or a metallic stay), etc. are warmed and therefore, it is a usual practice to lower the fixing temperature in conformity with the number of printed sheets.
When intermittent printing is effected, the pressure roller is heated by the remaining heat in each printing course and the heat thereof is accumulated therein and therefore, the temperature tends to become high as compared with that in continuous printing.
Also, the heater ON timing in the fixing process is generally such that the printer receives a printing signal so that fixing device constituting parts such as the pressure roller and the fixing film may sufficiently warm before the paper enters the fixing device at the start of printing, and the heater is turned on when the printing process starts.
At this time, it is usually the case that the heater is turned on after the switched-ON of a sheet feed sensor during the sheet feeding at the start of printing or after the switched-ON of a top sensor or a registration sensor before the paper enters the fixing nip.
In the case of intermittent printing in which the stop interval is short, as compared with continuous printing, the temperature of the pressure roller becomes excessively high (it will hereinafter be called the hot start to start printing in a state in which the pressure roller is warming), and by a temperature of about 150xc2x0 C. or higher being reached for the printing of about 50 sheets, the slip occurs.
About Condition (iii)
The slip is liable to occur in the case of printing patterns high in dot percentage. As image patterns high in dot percentage, mention may be made of a solid black image and a graphic pattern such as a halftone, and these patterns are such that a toner image is formed on the entire area of the printing surface of the paper. Therefore, during the occurrence of the slip, the steam created from the paper cannot escape because of the presence of the toner image on the printing surface and thus, positively escapes to the back side of the paper, i.e., the pressure roller side. Consequently, the slip is liable to occur due to the steam collecting between the pressure roller and the paper.
Also, the film type fixing device has suffered from the following problems.
In an image forming apparatus using the film type fixing device, the quantity of heat is liable to become deficient or excessively great depending on the environmental temperature at which the image forming apparatus is used because the heat capacity of the fixing device is small as previously described. In the film type fixing device, the quantity of heat to be given from the heater to a recording material (hereinafter referred to as the paper sheet) differs considerably depending on the temperature of the pressure roller which is a pressure member. Accordingly, the controlled temperature of the heater is suitably changed so that depending on the number of printed sheets and the time elapsed after the preceding fixing process has been terminated, a quantity of heat conforming to the temperature of the pressure roller may be given from the heater to the paper sheet so that the quantity of heat given from the heater and the pressure roller to the paper sheet may always be constant.
Specifically, in the case of continuous printing from when the fixing device after the switched-ON of a power source is cold, the temperature of the pressure roller is low and therefore the controlled temperature is set to a high level, and when the number of printed sheets is increased and the temperature of the pressure roller becomes high, a quantity of heat is also supplied from the pressure roller to the paper sheet and therefore, the controlled temperature is set so as to be gradually lowered.
When the continuous printing is terminated and the standby state is once restored, it is necessary to determine the next controlled temperature (target temperature) at a point of time whereat the next printing signal has come. If the set controlled temperature is too high, a portion of an image will remain on the fixing film side due to the excessively great quantity of heat and there will arise the problem of offset which will cause a bad image after one round of the film, and the problem of fixing slip in which the conveyance of the paper is hampered by steam created from the paper in the fixing nip portion. On the other hand, if the set fixing temperature is too low, bad fixing or the contamination of the pressure roller in which the toner offset to the film during fixing collects on the pressure roller may occur.
Usually, at the start of printing, the temperature of the temperature control thermistor of the fixing device is monitored as the temperature of the fixing device and the set controlled temperature is determined, but when the fixing device is still warm immediately after the printing operation, the determined temperature has a factor which becomes an error to the optimum temperature. That is, after a small amount of printing, the lowering of the temperature of the fixing device is sudden, but after a large amount of printing, the whole of the fixing device is sufficiently warm and therefore, the fall of the temperature is gentle. For example, if the controlled temperature of the thermistor of the fixing device is monitored to thereby determine the controlled temperature when the next print signal has come immediately after the termination of a small amount of printing, a high temperature is detected and thus, lower temperature control setting is selected. Actually, however, only a small amount has been printed and the fixing device is not yet sufficiently warmed and therefore becomes cold immediately after the determination of the controlled temperature, and may become deficient in quantity of heat and bad fixing may occur when the fixing operation for the next print is performed.
Due to such an error factor, a wrong controlled temperature may be selected and the problems as noted above may arise.
Also, the film type fixing device is small in heat capacity and therefore is also liable to be affected by the ambient temperature of the image forming apparatus. For example, if the atmospheric temperature of the image forming apparatus is low, the temperature of the paper, the toner, etc. also become low and therefore, the bad fixing and thee contamination of the pressure roller by the deficient quantity of heat are liable to occur. On the other hand, if the atmospheric temperature is high, the hot offset and the slip of the fixing device by an excessively great quantity of heat may be liable to occur.
When the film type fixing device is adopted in a medium or high speed apparatus, the fixing device passing time tends to become shorter correspondingly to the fact that the process speed becomes higher, and to become liable to be affected by the ambient temperature of the image forming apparatus. Also, the setting of the temperature control becomes high in order to secure the fixing property, and bad fixing and hot offset will occur unless an appropriate controlled temperature is adopted.
It is an object of the present invention to solve the above-noted problems.
It is another object of the present invention to prevent the above-described slip during intermittent printing and hot start in a fixing device of the film heating type and the pressure rotary member driving type and an image forming apparatus provided with the fixing device.
It is another object of the present invention to solve the above-noted problems in an image forming apparatus using a film type fixing device as a fixing device for fixing an unfixed image formed and borne on a recording material, that is, to achieve the optimization of the fixing control temperature in the hot started state of the fixing device, and to prevent the slip and hot offset of the fixing device.
In order to achieve the above objects, the fixing device and the image forming apparatus according to the present invention are as follows:
(1) A fixing device comprising a fixedly supported heater, film sliding relative to the heater, and a pressure rotary member brought into pressure contact with the heater with the film interposed therebetween and forming a fixing nip, and rotatively driven, and wherein a recording material having an unfixed image formed thereon is nipped and conveyed between the film and the pressure rotary member at the fixing nip to thereby fix the unfixed image by the heat of the heater, further having temperature control means for changing over the heating time of the heater when the temperature difference between the detected temperature by a temperature detecting element for detecting the temperature of the heater and the target heating temperature of the heater is smaller than a desired value.
More specifically, when at the start of printing, the temperature difference between the detected temperature by the temperature detecting element and the target heating temperature of the heater is Axc2x0 C. or less, control is effected such that the ON timing of the heater is delayed more than when the temperature difference is greater than Axc2x0 C.
That is, when the detected temperature by the temperature detecting element for detecting the temperature of the heater is defined as T1 and the target heating temperature of the heater is defined as T2, when T1xe2x88x92T2xe2x89xa6A, the ON timing of the heater is delayed more than when T1xe2x88x92T2 greater than A.
By the above-described construction, when intermittent printing has continued and the pressure roller has been sufficiently warmed by the remaining heat during the printing, T1xe2x88x92T2xe2x89xa6A is brought about at the start of the next printing, and raising is effected with the ON timing of the heater delayed more than when T1xe2x88x92T2 greater than A. Thus, the pressure roller is not excessively warmed at the starting of the heater, and even if a recording material (paper sheet) having absorbed moisture is nipped by the fixing nip, the steam created between the pressure roller which is a pressure rotary member and the paper is of a small amount, and even if the paper contacts with the pressure roller, it will not slip. Consequently, even in intermittent printing, the driving force of the pressure roller can be stably transmitted to the film through the paper sheet, and it becomes possible to prevent slip and stabilize the conveyance of the paper sheet.
Also, even if the ON timing of the heater is delayed, it becomes possible to raise the heater to the target heater temperature within a short time because the pressure roller, the film guide, etc. are sufficiently warm, and a temperature necessary for fixing can be sufficiently secured and bad fixing does not occur.
(2) The fixing device described in item (1) above, wherein the heating time of the heater is changed over a plurality of times in conformity with the temperature difference between the detected temperature by the temperature detecting element for detecting the temperature of the heater and the target heating temperature of the heater.
More specifically, control is effected such that when the temperature difference is A, B, and C [xc2x0 C.], and A greater than B greater than C, the ON timing of the heater is started late in the order of A, B, and C. By the above-described construction, the fixing temperature at the start of printing is Ta less than Tb less than Tc [xc2x0 C.], and even when Tc is used to secure the fixing property for thick paper or the like, the ON timing of the beater can be made later than Ta and Tb. Consequently, even when the temperature of the heater is made high and intermittent printing is continued by the use of thick paper or the like, the pressure roller is not excessively warmed and it becomes possible to suppress the amount of steam created from the paper at the fixing nip to thereby prevent slip and stabilize the conveyance of the paper. At the same time, the fixing temperature is also high for the thick paper and therefore, a sufficient fixing property can be secured.
(3) The fixing device described in item (1) above, having an atmospheric temperature detecting element and wherein the heating time and the target heating temperature of the heater are changed over a plurality of times in conformity with the detected temperature by the atmospheric temperature detecting element and the temperature difference between the detected temperature by the temperature detecting element for detecting the temperature of the heater and the target heating temperature of the heater.
More specifically, temperature control is effected such that when the detected atmospheric temperature is Txcex1, Txcex2 and Txcex3 (Txcex1 less than Txcex2 less than Txcex3), the ON timing of the heater is made later in the order of Txcex1, Txcex2 and Txcex3. By the above-described construction, Txcex1 can be judged to be a low-temperature environment, Txcex2 can be judged to be an ordinary environment, and Txcex3 can be judged to be a high-temperature environment, and the amount of steam created from the paper at the fixing nip is greater in the order of Txcex1, Txcex2 and Txcex3. So, by making the ON timing of the heater later in the order of Txcex1, Txcex2 and Txcex3, the pressure roller is not excessively warmed in each environment and it is possible to suppress the amount of created steam to thereby prevent the slip. In addition, in the low-temperature environment, the paper is cold and therefore the quantity of heat required for fixing becomes great and the fixing temperature becomes high and thus, much time is taken for raising, but the heater is also raised early and therefore, the time for raising is sufficient and the fixing property can be secured.
(4) An image forming apparatus having image forming process means for forming an unfixed image on a recording material, and a fixing device for heating and fixing the unfixed image on the recording material, wherein the fixing device is the fixing device described in any of items (1) to (3) above.
(5) The image forming apparatus described in item (4) above, wherein the image forming process means is transfer type electrophotographic process means.
According to the present invention, in a fixing device of the film heating type or the pressure rotary member driving type and an image forming apparatus provided with the fixing device, when the temperature difference between the temperature of the vicinity of the heater detected before printing and the fixing target temperature is smaller than a desired value, the raising timing of the heater is made late, whereby the slip caused by the pressure roller being excessively warmed during intermittent printing can be prevented.
Also, by the raising timing of the heater being changed in conformity with the temperature difference, it becomes possible to prevent the slip irrespective of the fixing temperature for thick paper or the like, and also secure the fixing property.
Good fixed images can be obtained irrespective of the environment, the sheet type and the situation in which the paper is left, and also it becomes possible to prevent the slip and stabilize the conveyance of the fixed paper.
Further, by disposing an atmospheric temperature sensor (environment detecting sensor) in the image forming apparatus, optimum fixing temperature control conforming to the environment of use can be realized, and good fixed images can be obtained irrespective of the environment, the sheet type and the situation in which the paper is left, and also the slip can be prevented and the stabilization of the conveyance of the fixed paper can be realized.
An image forming apparatus using another film type fixing device according to the present invention is as follows:
(1) An image forming apparatus having a fixing device having a heater, film sliding relative to the heater, and a pressure member brought into pressure contact with the heater with the film interposed therebetween to thereby form a fixing nip, and wherein a recording material having an unfixed image formed thereon is nipped and convoyed between the film and the pressure member at the fixing nip to thereby fix the unfixed image by the heat of the heater, and in the next printing after the lapse of a predetermined time after the termination of any printing operation, the target temperature of the fixing device during the initial rotation in the printing operation and the heater electrical energization timing are changed in conformity with the temperature of the fixing device.
That is, in the printing in the state after the termination of any printing operation in which the fixing device is warm, the target temperature (ready temperature) during the initial rotation in the printing operation and the heater electrical energization timing are changed in conformity with the temperature of the fixing device after the lapse of a predetermined time, and specifically the temperature of the heater.
If immediately after the termination of printing, the fall of the temperature of the fixing device is sudden and therefore, the determination of the controlled temperature for the next printing is liable to have an error, but when a predetermined time (the order of several tens of seconds to one minute) elapses, the fall of the temperature becomes gentle and therefore, the temperature of the fixing device can be measured in a stable state. For example, as a method of changing the target temperature during the initial rotation in the printing operation and the heater electrical energization timing, such control as to set the target temperature to a high level because the fixing device is cold if the temperature after the lapse of a predetermined time after printing has been terminated is a predetermined temperature or lower, and to delay the timing for starting to electrically energize the heater for the prevention of the excessive rise in the temperature of the fixing device because the fixing device is hot if the temperature is higher than the predetermined temperature can be effected. Thereby, appropriate temperature control can be effected even during the printing operation in a state in which the fixing device is warm after the termination of the preceding printing operation, and hot offset and the slip of the fixing device can be prevented.
If conversely, the temperature is the predetermined temperature or lower, the fixing device is cold and therefore, target temperature during the initial rotation in the printing operation can be heightened. It is because the pressure roller can be more warmed within a limited fixing device raising time and bad fixing and the contamination of the pressure roller can e prevented that the target temperature during the initial rotation in the printing operation is set to a higher level than the fixing temperature when the paper is actually fixed.
(2) Also, an image forming apparatus having a fixing device having a heater, film sliding relative to the heater, and a pressure member brought into pressure contact with the heater with the film interposed therebetween to thereby form a fixing nip, and wherein a recording material having an unfixed image formed thereon is nipped and conveyed between the film and the pressure member at the fixing nip to thereby fix the unfixed image by the heat of the heater, further having an atmospheric temperature sensor for detecting the temperature of the atmosphere around the image forming apparatus, and wherein when the next printing is to be effected after the lapse of a predetermined time after the termination of any printing operation, the target temperature of the fixing device during the initial rotation in the printing operation and the heater electrical energization timing are changed in conformity with the atmospheric temperature detected by the atmospheric temperature sensor and the temperature of the fixing device.
That is, the image forming apparatus has an atmospheric temperature detecting sensor for detecting the temperature of the atmosphere around the image forming apparatus), and the target temperature during the initial rotation in the printing operation and the heater electrical energization timing are changed in conformity with body of the atmospheric temperature detected by the atmospheric temperature detecting sensor and the temperature of the fixing device after the lapse of the aforementioned predetermine time.
Because of the data of the atmospheric temperature, the setting of the target temperature during the initial rotation in the printing operation and the heater electrical energization timing can be effected more finely and accurately. For example, when the atmospheric temperature is high, the paper fed is warm and therefore the heater electrically energizing timing is delayed. Thereby, hot offset and the slip of the fixing device are reliably prevented. When the atmospheric temperature is low, the paper fed is cold and therefore, the target temperature can be changed to a high temperature to thereby prevent bad fixing.
According to the present invention, the temperature of the fixing device is monitored in a predetermined time after the termination of any printing operation, and the fixing device raising method, i.e., the target temperature (ready temperature) during the initial rotation in the printing operation and the heater electrical energization timing are changed in conformity with that temperature to thereby optimize the controlled temperature in the next printing, and prevent hot offset and the slip of the fixing device as well as bad fixing and the contamination of the pressure roller.
Further, by having the atmospheric temperature sensor and also controlling the detected data, there is the effect of being capable of determining the next controlled temperature more precisely.